Luxembourg
When the Grand Duchy of Luxembourg found itself without a male heir at the beginning of the 20th century, the morganatic counts von Merenberg proposed themselves as heirs, being the last legitimate descendants in the male line of the House of Nassau. Grand Duke William IV, however, chose to confirm the law of succession stipulated in 1815 by the Congress of Vienna to allow a female descendant in the Nassau male line to become successor to the throne (his own daughter Marie-Adélaïde) instead.
Read more about this topic: Morganatic Marriage